1. Technical Field
The present invention relates to a heat sink useful for optimizing heat dissipation from electronic components. More particularly, the present invention relates to a multi-element heat sink with improved characteristics for dissipating heat from electronic components such as power amplifiers so that the components are reduced to a lower temperature state.
2. Background Art
With the advancement of electronic devices being produced for many different applications, for many different locations, cooling electronic devices have become a substantial industry. For instance, in devices such as power amplifiers for tower-mounted infrastructures, cooling is essential to maintain the effective operation of the device. Moreover, the performance of components of computing devices decreases unless sufficient thermal energy is removed from the unit. Semiconductors also suffer in that their performance deteriorates when the operating temperature increases to an undesired level around the semiconductor device.
The traditional method of cooling an electronic device is by using a fan to circulate air around the electronic device, and thus remove thermal energy. Unfortunately, the problems associated with fan-type technology are multiple, including a relatively low cooling efficiency, a bulky power source and a limited functional size. Furthermore, in applications such as tower mounted infrastructures, fan-type technology is undesirable because of the weight of fan technology and further because fan failure can have extremely disadvantageous effects. Thus, passive cooling is most desired in such applications. Methods have developed to replace fan-type cooling devices which are both smaller and located in the immediate vicinity of the electronic device needing to be cooled. One purpose of the present invention is to provide a heat management device which can eliminate the use of fans, but more generally, it is to provide a more efficient heat sink for use in situations where heat sink performance is limited by spreading in the base of the heat sink, allowing reduction of airflow requirements or lower thermal resistance for a given airflow or in natural convection.
One popular method for the dissipation of heat from modern electronics is an aluminum heat sink. While aluminum does not have the thermal conductivity of copper, it is significantly lighter in weight (having a density of about 2.7 g/cc as opposed to 8.9 g/cc for pure copper). Thus, for portable applications like laptop computers or stationary applications where weight supporting structures are undesirable, aluminum has been preferred. In addition, aluminum heat sinks tend to be substantially less expensive than copper heat sinks.
Generally, aluminum heat sinks are formed by one of two methods, extrusion and die casting. Extruded aluminum heat sinks are produced by forcing molten aluminum through a precision die to produce an article of constant cross-section. Extruded aluminum heat sinks have a thermal conductivity of about 220 W/m-K, which is useful for many thermal dissipation applications. However, extruded aluminum heat sinks are disadvantageous when the non-finned surface of the heat source has a complex shape. In other words, when it is desired to position the heat sink such that it sits on multiple components, the surface of the heat sink mating with the electronic components must conform to the differing profiles of the various components. A heat sink with a flat mating profile would not be adequate, but machining or otherwise forming a complex profile into an extruded aluminum heat sink is an expensive and time-consuming exercise.
Heat sinks formed of die cast aluminum or magnesium (or alloys thereof) can solve this problem, because die casting can be used to form a heat sink with a complex surface pattern. Die cast aluminum heat sinks are formed by injecting molten aluminum into a steel mold or die under high pressure. While the use of die cast aluminum heat sinks can provide a mating surface having complex shapes in a cost effective manner, the thermal conductivity of cast aluminum heat sinks is often only about 100 W/m-K or lower, much less than desirable for most electronic thermal management application and less than half that of extruded aluminum heat sinks.
What is desirable, therefore, is a way to leverage the relatively high thermal conductivity of extruded aluminum heat sinks with the flexibility of shape of die cast aluminum heat sinks.
An additional method of managing thermal energy is through graphite-based components which offer thermal conductivity comparable with or better than copper or aluminum but at a fraction of the weight while providing significantly greater design flexibility. Graphite-based thermal management products take advantage of the highly directional properties of graphite to move heat away from electronic components while having thermal conductivities substantially higher than typical aluminum alloys used for heat management. Furthermore, graphite is anisotropic making it more suitable for channeling heat in a preferred direction.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional such as thermal and electrical conductivity.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal conductivity due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from high compression, making it especially useful in heat spreading applications. Sheet material thus produced has excellent flexibility, good strength and a high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cc to about 2.0 g/cc.
The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon compression of the sheet material to increase orientation. In compressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.